Changing the shape of a window (animation)

["Cyr, Pierre (EXP)" <pierre cyr lmco com>]

Hello,

I have a GTK application that has a shaped main window (based on the
wheelbarrow example).  It's pretty close to a circle.

What I would like to do is, after a button click, slide (animate) another
pixmap from underneath the main window to the right of my circle so that it
would look something like this.
  ______
 / \    \
|   |    |
 \-/----/


I need to change the mask of my main window to include part of the sliding
pixmap as well as the original main window.

I have tried different combination of the gdk_window_merge_child_shapes and
gdk_window_set_child_shapes, but I get weird shapes as result.

Is there any easy way that I am not thinking of to do something like this?

Thanks,